1. Field of the Invention
The invention relates to a receiving circuit, more specifically, to a receiving circuit and a receiving method that receive signals generated by modulating carrier waves having the substantially same frequency according to digital signals output from a plurality of systems.
2. Description of the Related Art
In recent years, the wireless smart key system has become widely available. In the wireless smart key system, a driver operates the portable device or the portable device responds to a request made by the vehicle-mounted device so as to lock or unlock the doors of the vehicle or to turn on the ignition switch to start the engine. The wireless smart key system uses a carrier wave in the frequency band of 300 MHz.
A tire pressure monitoring system (hereinafter abbreviated as “TPMS”) monitors the tire pressure of the four wheels of the vehicle, and the pressure of a spare tire. TPMS is required by law in the United States, and therefore it is expected that TPMS will become further widespread in near future. This system also uses a carrier wave in the frequency band of 300 MHz.
Both systems use the radio waves in the frequency band of 300 MHz. However, the timings at which the radio waves are transmitted from the both systems are asynchronous, and radio interference may possibly occur. Accordingly, the frequency of the radio wave transmitted from the wireless smart key system is made different from the frequency of the radio wave transmitted from TPMS by several MHz to several tens of MHz so that the radio interference does not occur. In other words, a receiver is provided for each system.
Japanese Patent Application Publication No. JP-A-2005-236556 describes a receiver that receives radio waves from a remote key, a smart key, and tire pressure detection means, and extracts the baseband signals of the received radio waves. Then, the receiver demodulates the extracted baseband signals through the low-pass filter whose cut-off frequency is changed in accordance with a command signal, so that the desired signal is output.
Further, Japanese Patent Application Publication No. JP-A-2004-244851 describes a keyless receiving unit that can receive signals from the keyless entry system and signals from the tire pressure monitor. In addition, Japanese Patent Application Publication No. JP-A-2003-72330 describes a receiver for the keyless entry system that receives signals from the tire pressure sensor as well as signals from the keyless entry system.
The carrier waves used by the wireless smart key system and TPMS need to have the substantially same frequency so that one analog receiver can receive the signals from the both systems. In this case, however, radio wave interference occurs.
The timings at which the wireless smart key system and TPMS transmit signals are asynchronous. Also, the electric field strength of the signals at the analog receiver changes depending on the situation. Therefore, a problem arises that the receiver can receive signals from one of the systems, which have higher electric field strength, but cannot receive signals from the other system, which have lower electric field strength.
For example, in the wireless smart key system, the electric field strength of the signals at the analog receiver changes depending on where the user is. In TPMS, the transmitter is installed on a tire valve of each of the tires. Therefore, the positions of the transmitters when the transmitters transmit signals change as the tires rotate. Also, the electric field strength of the signals at the analog receiver changes depending on the reflectance of a road surface, and depending on the existence of reflective objects around the vehicle.
Therefore, whether the signals from the wireless smart key system or the signals from TPMS have higher electric field strength at the receiver depends on the situation. In particular, if the wireless smart key system cannot communicate when needed, the wireless smart key system cannot be operated as expected by the user. This makes the user feel unsatisfied.
In the receiver described in Japanese Patent Application Publication No. JP-A-2005-236556, the cut-off frequency of the low-pass filter is changed in accordance with the command signal. Thus, when signals from the smart key and signals from the tire pressure detection means are received at the same time, the receiver can receive only the signals from one of the system, which have higher electric field strength.
When the frequency of the signals from the smart key is set to 700 Hz and the frequency of the signals from TPMS is set to 5 KHz as shown in FIG. 1A, the frequency of the signals from TPMS (e.g. 5 KHz) is significantly attenuated as shown in FIG. 1B if the cut-off frequency of the low-pass filter is set to 1 KHz so that the signals from the smart key pass through the filter.
In contrast, if the cut-off frequency of the low-pass filter is set to 5.2 KHz so that the signals from TPMS pass through the filter as shown in FIG. 1C, the frequency of the signals from the smart key is not attenuated and mixed with the signals from TPMS as noise.